1. Field of the Invention
The present invention relates to a scanning mechanism for a radiation monitoring TV set for remotely and automatically scanning and imaging the distribution state of a radioactive material with a directional radiation detector.
2. Description of the Related Art
In a nuclear power facility, a facility utilizing a radioisotope, or the like, it is necessary to carry out exposure control for workers by detecting a leakage direction, an intensity, a dose rate or the like of radiation (particularly, gamma rays) emitted from an equipment or a pipe and to prepare a radiation map or the like. The preparation of the radiation map for this control/administration can be made by a radiation TV set disclosed in, for example, the publication of Japanese Patent Application Laid-open No. 227186/1985. In this disclosed radiation TV set, the intensity and the distribution state of radioactivity from a radiation source are calculated by the use of a computer on the basis of information data regarding radiation obtained by scanning an observation object range with a directional gamma-ray detector and distance data obtained by observing an observation object with an optical range finder without manual intervention, and a color-divided radioactivity distribution image is formed in accordance to the intensity of the radioactivity. Then, this image is synthesized with an optical image of the observation object to display the synthesized image on a CRT.
In a conventional radiation TV set, it is assumed that observation is performed, putting the TV set in front of an observation object, and the range to be scanned by a directional gamma-ray detector is assumed to be a comparatively small range within the field of view caught by an optical-image camera tube in the front direction of the TV set. The conventional radiation TV set employs a scanning mechanism shown in FIGS. 2a and 2b in which vertical and horizontal pivots are not independent of each other, as a scanning mechanism for scanning an observation object range by vertically and horizontally rotating the orienting direction of the directional gamma-ray detector. Here, FIG. 2a shows the front of the scanning mechanism, and FIG. 2b shows a sectional view of the scanning mechanism in FIG. 2a along the line A-B in FIG. 2a.
In the scanning mechanism shown in these drawings, a gamma-ray detection probe 13 shown in FIG. 2b is received in a gamma-ray shielding vessel 12 made of a high-density metal such as lead, and this vessel 12 is equipped with a collimator 11 similarly made of a high-density metal such as lead and provided with a pore in an orienting direction, whereby a directional gamma-ray detector 1 shown in FIG. 2a is constituted. The directional gamma-ray detector 1 is attached to a vertically rotating frame 6 having a rotary shaft 61 on a line passing through the sensitivity center of the detector, and the rotary shaft 61 of the vertically rotating frame 6 is rotatably held by a horizontally rotating frame 7. The rotary shaft 61 can be driven by a vertically rotating frame driving motor 62 connected to the shaft 61.
A rotary shaft 71 is attached to the horizontally rotating frame 7 so as to cross at right ankles with the rotary shaft 61 of the vertically rotating frame 6. This rotary shaft 71 is rotatably held by a support rod 5, and torque is transmitted to the shaft 71 via a power transmission device 73 by a horizontally rotating frame driving motor 72 secured to the support rod 5.
The scanning operation of the scanning mechanism is performed by controlling the rotation of the vertically and horizontally rotating frame driving motors 62 and 72 in accordance with a command signal sent from a control section of the radiation TV set. However, a scanning range .PHI., as shown in FIG. 3, depends upon an angle .theta. between the orienting direction of the directional gamma-ray detector 1 and the direction of the rotary shaft 71 of the horizontally rotating frame 7 and changes in accordance with the equation (1) to the same rotation angle .psi. of the rotary shaft 71: EQU .PHI.=.psi.sin .theta. (1)
When the angle .theta. is about 90.degree., the value .PHI. becomes almost equal to .psi. because of the nature of the sinusoidal function sin .theta.. However, when the angle .theta. deviates largely from 90.degree., scanning is carried out in a restricted range in view of the same rotation angle of the rotary shaft 71 of the horizontal scanning detector rotating frame 7. In an extreme case in which the direction of the detector coincides with the rotary shaft, i.e., .theta.=0, the scanning cannot be performed, depending upon the rotation.
In a conventional radiation TV set which is arranged in front of an observation object to scan radiation within the field of view caught by an optical-image camera tube, the scanning range is not so large and the central direction of the vertical scanning is 90.degree. to the horizontal scanning rotary shaft. Therefore, the scanning range of a small angle around the direction of 90.degree. is necessary and sufficient.
In a conventional radiation TV set in the above-mentioned background, there is employed a scanning system of a relatively simple structure in which the rotation of the vertical scanning follows the horizontal scanning rotary shaft.